Many electronic devices or loads driven by DC power require one or more stable DC supply voltages for operation. These DC supply voltages are usually obtained with the aid of AC-DC converters which employ typical transformers and rectifiers as well as suitable capacitors and filters to convert an AC supply voltage to a determined DC voltage. More complicated electronic loads, such as data processing units and logic circuits are more refined as to their voltage requirements. For example, processing circuits may process vastly different amounts of data at various points in time. This means that their workloads and hence energy requirements vary significantly. Such loads would benefit greatly from an adjustable and well-defined DC supply voltage by reducing their power consumption.
Prior art techniques for deriving a variable DC output voltage V.sub.OUT from a DC supply voltage V.sub.DD for use by variable loads have been proposed. The article by L. Nielsen et al. entitled "Low-Power Operation Using Self-Timed Circuits and Adaptive Scaling of Power Supply Voltage", IEEE Transaction on VLSI Systems, December 1995, pp. 391-397 proposes an approach to dynamically adjust the supply voltage based on the processing load. A similar idea for tracking temperature and process variations using a dynamic switching regulator has been proposed by M. Horowitz in "Low Power Processor Design Using Self-Clocking", Workshop on Low-Power Electronics, 1993.
Both of these ideas rely on using a DC--DC switching regulator capable of delivering an output voltage as required for operating the load. For example, when the load is a processor operating at a specific clock rate and the incoming data requires processing at a different rate, a desired clock rate is sent to the DC--DC converter. The converter then appropriately varies the supply voltage of both the processor and a ring oscillator so that the frequency of the ring oscillator matches the desired clock rate. When operating at a fixed processing rate, this type of DC--DC converter tracks the delay variations due to temperature and process change to operate at the lowest voltage level required.
Unfortunately, these types of prior art DC--DC switching regulators are not very responsive--they have a slow tracking speed. As a result, such regulators can not be effectively employed in the energy-on-demand paradigm.
The prior art also teaches the use of controllers to maintain feedback stability of DC--DC regulators. The best known strategy is the lead-lag compensation technique.
Implementation of this type of compensation becomes very difficult because of the wide range of operating conditions that the regulator must handle. Depending on the load, output voltage and switching frequency the location of the poles and zeroes move by substantial amount. To maintain stability with fast response time, the regulator needs to quickly compensate for unstable poles. Such complex regulators require an enormous amount of computational power, which for the low-power applications involves a large fraction of the total power delivered, thus greatly reducing power efficiency and defeating the purpose of the regulator.
A practical variable voltage DC--DC regulator has to be simple to implement and operate reliably over a wide range of output loads and supply voltages, i.e., in the low-power regime and any other power regime required. In addition, it has to have a fast tracking speed.